The RH sensor has an accuracy of ±2% RH through the temperature range 0 to 80°C. FP is a nonlinear function of only two things: RH and Temperature. If we know the RH at a given temperature then we can calculate dew point (or frost point if below freezing). If the sensor is reading 1% RH at 21°C then the frost point will be -38°C. If RH is 0.5% at 21°C then FP is -44°C. FP values are calculated using the August-Roche-Magnus approximation.

The argon sensor is heated to 120-130°F so over time this heat will migrate to the RH sensor and slightly skew the RH, FP, and temperature readings. The horizontal slot cut into the circuit board helps with this heat migration but doesn’t entirely prevent it.